It is well known that a technology which expands the sensitive wavelength region to the longer wavelength side by adding some kind of cyanine dye to silver halide photographic emulsions, i.e., spectral sensitization technology can be applied as one technology for the production of photosensitive materials. It is also known that the sensitivity obtained by spectral sensitization, i.e., spectral sensitivity is influenced by the chemical structure of sensitizing dyes and the properties of emulsion such as the halogen composition of silver halide, crystal habit, crystal system, silver ion concentration, hydrogen ion concentration, etc. Furthermore, this spectral sensitivity is also influenced by the additives for photography such as anti-fogging agent, coating auxiliary, sedimentation agent, color coupler, hardening agent, etc.
In general, a single sensitizing dye is employed to sensitize a definite spectral wavelength region in photosensitive materials. On the other hand, it is known that the efficiency of spectral sensitization remarkably increases when some kind of specifically selected dye or other organic substance exists other than the afore-mentioned dye. This effect is known as supersensitization. The supersensitization is a specific phenomenon because the addition of an additional dye or organic substance generally does not enhance the sensitivity and rather diminishes the sensitivity. Thus, extremely high selectivity is required of the organic substance or additional sensitizing dye which is used in this combination. Accordingly, an apparently small change in the chemical structure markedly influences this supersensitization. Therefore, it is difficult to obtain a suitable combination of compounds for use in supersensitization by a mere inference from the chemical structures thereof.
It is primarily important for the sensitizing dye which is used in the application of the supersensitization to the silver halide photographic emulsion to provide high spectral sensitivity. Secondly, strong sensitization of a specific narrow wavelength region is desired. Especially, in spectral sensitization at the green-sensitive region, it is necessary to provide higher sensitivity in a narrow wavelength region because the expansion of spectral sensitivity to the longer wavelength side and shorter wavelength side causes overlapping with the red-sensitive region and blue-sensitive region. This results in much color mixing, and in some cases, enhances the sensitivity to safety light which makes it difficult to handle.
Although sensitizing dyes which give the spectral sensitization called J-band are usually employed for this purpose, it is desired to give higher green-sensitivity without expanding the wavelength region of this spectral sensitivity, favorably with narrowing the wavelength region.
Furthermore, it is required that sensitizing dyes to be employed must not cause any undesirable interaction with color couplers or other photographic additives other than sensitizing dyes. Further, it is important to maintain stable photographic properties during the preservation of photosensitive materials It is also required for the sensitizing dyes that treated photosensitive materials containing them do not suffer residual coloration which may be caused by the sensitizing dyes. Especially, no residual coloration should occur for short period (usually several seconds to tens of seconds) treatment such as rapid treatment.
A further requirement is that the sensitizing dyes cause little fogging.
The combinations of the dyes which specially show supersensitization at the green-sensitive region are described in U.S. Pat. Nos. 3,580,724, 3,729,319, 3,397,060, etc. However, the technologies disclosed in these references are insufficient for obtaining photosensitive materials which have high green-sensitivity with little fogging, good preservation stability and little residual coloration.